1. Field of the Invention
The present invention relates to a non-aqueous electrolyte secondary battery, and more particularly, to a non-aqueous electrolyte secondary battery having excellent battery characteristics such as high-temperature cycle characteristics and overcharge characteristics.
2. Background Art
Portable information terminals such as portable telephones and notebook personal computers are becoming increasingly sophisticated and reduced in size and weight in recent years. These terminals use as portable power supplies non-aqueous electrolyte secondary batteries such as lithium-ion secondary batteries with high energy density and high capacity.
These secondary batteries use a non-aqueous electrolyte containing a non-aqueous solvent which contains carbonates such as ethylene carbonate (EC) or propylene carbonate (PC). These solvents, however, have poor stability during overcharging, and hence batteries using them do not have good overcharge characteristics.
It is possible to improve the safety against overcharging by making the non-aqueous carbonate-based solvent contain a carboxylic acid ester which is more resistant to charge and discharge than the solvent.
Carboxylic acid esters, however, weaken the negative electrode film, which is formed on the negative electrode surface during charge and discharge, thereby decreasing the high-temperature cycle characteristics of the batteries. Therefore, when the batteries are subjected to repeated charge-discharge cycles at temperatures higher than 60° C., their battery capacities decrease substantially.
Patent Document 1 discloses a lithium secondary battery with an electrolyte containing a cyclic carbonate and/or a cyclic ester, an alkyne derivative, and a tertiary carboxylic acid ester. This document states in paragraph 0004 that carboxylic acid esters, which have lower boiling points than chain carbonates, easily cause battery swelling and other problems during evaporation.
Patent Documents 2 and 3, on the other hand, state that adding an aliphatic nitrile compound to a non-aqueous electrolyte provides beneficial effects such as preventing accidental battery ignition, and reducing an increase in the viscosity of the electrolytic solution and a decrease in low-temperature performance. Patent Documents 2 and 3, however, do not mention a combination of a carboxylic acid ester and a nitrile compound.
Patent Document 4, on the other hand, discloses in paragraphs 0050 and 0051 a non-aqueous electrolyte secondary battery which has a non-aqueous electrolytic solution containing a carboxylic acid ester (n-butyl pivalate) and adiponitrile. This document, however, is not a technique focused on decreasing the shutdown response speed of a separator or the overcharge characteristics after many repeated cycles at high temperatures.    Patent Document 1: Japanese Patent Unexamined Publication No. 2003-59529    Patent Document 2: Japanese Translation of PCT Publication No. 2007-510270    Patent Document 3: Japanese Translation of PCT Publication No. 2007-519186    Patent Document 4: Japanese Patent Unexamined Publication No. 2004-179146